1. Field of the Invention
The present invention relates to an optical element used in optical information processing or optical communications or the like to correct aberrations, and also relates to an optical head and an optical recording/reproduction apparatus.
2. Related Art of the Invention
In recent years, the Digital Versatile Disc (DVD) has been attracting attention as a large capacity optical recording medium since it can record digital information at a recording density about seven times higher than the Compact Disc (CD).
In order to play back high density DVDs, the wavelength of the light source is made shorter than that for the CD (650 nm for DVD compared with 780 nm for CD), and the numerical aperture of the objective lens is made larger than that for the CD (0.6 for DVD compared with 0.45 for CD).
However, since the wavelength is made shorter and the lens NA is increased, as described above, wavefront aberration, especially coma, is generated because of the displacement (tilt) from the normal relative to the optical axis caused by warping or other irregularities of the optical disc, and thus the margin for tilt is smaller than in the case of CD.
To overcome this problem, an optical head which corrects wavefront aberration by using a liquid crystal panel is proposed in Japanese Laid-open No. 9-128785. However, the optical head described in the above cited patent publication can only correct one kind of aberration, for example, the coma caused by radial tilt.
Actual optical discs, however, may become warped in both radial and tangential directions, depending on the manufacturing method used, and the optical head described in the above cited patent publication cannot provide a sufficient correction effect. In view of this problem, an optical head which simultaneously corrects for the radial and tangential directions is proposed in Japanese Laid-open No. 10-247330.
One example of the above prior art optical head will be described with reference to drawings. FIG. 20 is a diagram showing the construction of an optical head (also called an optical pickup) according to the prior art. As shown in the figure, the optical head comprises a light source 170, a half-silvered mirror 171, a liquid crystal panel 172, an objective lens 173, an optical disc 174, a converging lens 175, a photodetector 176, a first tilt sensor 177, a second tilt sensor 178, and a liquid crystal panel control circuit 179.
The light source 170 is constructed, for example, from a semiconductor laser device, and outputs recording/playback coherent light directed to a recording layer of the optical disc 174. The half-silvered mirror 171 is a device for separating light; the liquid crystal panel 172 is a device that has the structure shown in FIG. 21 and the pattern shown in FIG. 22, and that corrects aberrations by applying a desired voltage to each electrode and thereby changing the refractive index of the liquid crystal and, hence, the phase of each electrode (the details will be described later); the objective lens 173 is a lens for focusing the light onto the recording layer of the optical disc 174; the converging lens 175 is a lens for focusing the light, reflected from the recording layer of the optical disc 174, onto the photodetector 176; the photodetector 176 is a device that receives the light reflected from the recording layer of the optical disc, and converts the light into an electrical signal; the first tilt sensor 177 is a sensor which detects the radial tilt angle of the optical disc 174, and generates a signal proportional to the detected tilt angle; the second tilt sensor 178 is a sensor which detects the tangential tilt angle of the optical disc 174, and generates a signal proportional to the detected tilt angle; and the liquid crystal panel control circuit 179 is a circuit that generates two signals, one for controlling each electrode of the liquid crystal panel according to the signal generated by the first tilt sensor 177 and the other for controlling each electrode of the liquid crystal panel according to the signal generated by the second tilt sensor 178.
The operation of the thus constructed optical head will be described below. Linearly polarized light emitted from the light source 170 is reflected by the half-silvered mirror 171, and the light thus redirected in the direction of the optical disc 174 enters the liquid crystal panel 172. Suppose here that the optical disc 174 is rotating with the disc tilted from the normal in the radial direction relative to the optical axis; then, the first tilt sensor 177 outputs a signal proportional to the amount of the tilt (the radial tilt angle), and the signal is input to the liquid crystal panel control circuit 179. The liquid crystal panel control circuit 179 then outputs the necessary signal to each electrode portion of the pattern shown in FIG. 22 of the radial tilt correcting electrode shown in FIG. 21 to generate a wavefront aberration that compensates for the wavefront aberration caused when the optical disc 174 is tilted in the radial direction, and this signal is input to the liquid crystal panel 172.
If, at the same time, the optical disc 174 is tilted from the normal in the tangential direction relative to the optical axis, the second tilt sensor 178 outputs a signal proportional to the amount of the tilt (the tangential tilt angle), and the signal is input to the liquid crystal panel control circuit 179; the liquid crystal panel control circuit 179 then outputs the necessary signal to each electrode portion of the pattern shown in FIG. 22 of the tangential tilt correcting electrode shown in FIG. 21 to generate a wavefront aberration that compensates for the wavefront aberration caused when the optical disc 174 is tilted in the tangential direction, and this signal is input to the liquid crystal panel 172.
As a result, the light entering the liquid crystal panel 172, as it passes through the liquid crystal panel 172, is given such wavefront aberrations that compensate for the wavefront aberrations caused when the optical disc is tilted in both the radial and tangential directions. The light passed through the liquid crystal panel 172 is then focused onto the optical disc 174 by means of the objective lens 173.
Since the light having wavefront aberrations that compensate for the wavefront aberrations caused when the optical disc 174 is tilted is focused through the objective lens, a light spot free from aberrations and thus focused to the diffraction limit is formed on the optical disc. Next, the light reflected from the optical disc 174 emerges with wavefront aberrations proportional to the tilt of the optical disc 174, but these wavefront aberrations are corrected by the liquid crystal panel 172.
The light passed through the liquid crystal panel 172 is not directed back to the light source 170, but is passed through the half-silvered mirror 171 and directed to the converging lens 175 which focuses the light onto the photodetector 176. The photodetector 176 outputs a focus error signal indicating the focusing condition of the light on the optical disc 174, and also outputs a tracking error signal indicating the position of the light shone thereon.
One of these signals is supplied to focus control means not shown and, based on the focus error signal, the focus control means controls the position of the objective lens 173 along its optical axis so that the light is always kept in focus on the optical disc 174.
On the other hand, tracking control means not shown controls the position of the objective lens 173 based on the tracking error signal so that the light is kept focused on the desired track on the optical disc 174. The photodetector 176 also reproduces information recorded on the optical disc 174.
The operating principles of the liquid crystal panel 172 and tilt corrections will be described in detail below. First, FIG. 23 shows the wavefront aberration distribution on the recording surface of the optical disc 174 at the best focal point when the radial tilt angle of the optical disc is 1 degree, the NA of the objective lens is 0.6, and the substrate thickness of the optical disc 174 is 0.6 mm.
Here, if a phase that would perfectly correct for this distribution is given to the light, the light spot on the optical disc will be focused to the diffraction limit even when the optical disc is tilted. The wavefront aberration distribution in the case of tangential tilt will be given by rotating through 90 degrees the wavefront aberration distribution obtained when radial tilt exists. In that case also, a phase that would perfectly correct for this distribution should be given.
Next, a method of correcting for the wavefront aberration distribution of FIG. 23 will be described. When the optical disc is tilted, there occurs a phase distribution such as shown in FIG. 23, and then such phase distribution that can cancel the phase distribution shown in FIG. 23 should be given to the light. That is, the optical path length should be selectively changed.
Since the liquid crystal is capable of varying its refractive index in accordance with an externally applied voltage, the optical path length can be selectively changed by applying an external voltage. That is, the liquid crystal exhibits the property called birefringence in which the refractive index differs between the alignment direction of the liquid crystal and the direction normal to it, as shown in FIG. 32.
Accordingly, by forming the pattern shown in FIG. 22 and by applying the necessary voltage to each region, the phase distribution shown in FIG. 23 can be perfectly corrected.
To make the above correction possible, the liquid crystal panel has the structure shown in FIG. 21. In FIG. 21, reference numeral 180 is first glass, 181 is a radial tilt correcting transparent electrode, 182 is a radial tilt correcting liquid crystal, 183 is a common transparent electrode, 184 is second glass, 185 is a tangential tilt correcting liquid crystal, 186 is a tangential tilt correcting transparent electrode, and 187 is third glass.
Next, the tilt correcting method will be described with reference to the structural diagram of FIG. 21. First, to correct for radial tilt, a prescribed voltage is applied to the uniformly formed common transparent electrode 183 (the voltage applied here is AC and its rms value is expressed by V), and different voltages are applied to the respective regions of the radial tilt correcting transparent electrode, generating different electric fields between the common transparent electrode 183 and the respective regions, thereby selectively changing the refractive index of the radial tilt correcting liquid crystal 182 and thus selectively changing the phase to correct for the radial tilt.
Next, to correct for tangential tilt, a prescribed voltage is applied to the uniformly formed common transparent electrode 183 (the voltage applied here is AC and its rms value is expressed by V), and different voltages are applied to the respective regions of the tangential tilt correcting transparent electrode 186, generating different electric fields between the common electrode and the respective regions, thereby selectively changing the refractive index of the tangential tilt correcting liquid crystal 185 and thus selectively changing the phase to correct for the tangential tilt.
When simultaneously correcting for the radial and tangential tilts, a prescribed voltage is applied to the uniformly formed common transparent electrode 183 (the voltage applied here is AC and its rms value is expressed by V), and voltages are applied to the respective regions of the radial tilt correcting transparent electrode, generating different electric fields between the common transparent electrode 183 and the respective regions, thereby selectively changing the refractive index of the radial tilt correcting liquid crystal 182 and thus selectively changing the phase from to correct for the radial tilt, while at the same time, voltages are applied to the respective regions of the tangential tilt correcting transparent electrode 186, generating different electric fields between the common electrode and the respective regions, thereby selectively changing the refractive index of the tangential tilt correcting liquid crystal 185 and thus selectively changing the phase to correct for the tangential tilt.
In the optical head of the above construction, however, since the tilt correcting liquid crystal panel requires three glass sheets and two different liquid crystal layers to correct for radial and tangential tilts, respectively, the thickness of the liquid crystal panel increases, making it difficult to make the optical head compact and thin in construction and increases the cost.
Furthermore, the center of the radial tilt correcting transparent electrode and the center of the tangential tilt correcting transparent electrode must be aligned with respect to each other, but since the two electrodes are separated by a glass sheet increasing the distance between the patterns to be aligned, it is difficult to align the patterns with each other and pattern displacements tend to occur, resulting in the degradation of the aberration correcting effect.
The prior art optical head has a further problem as described below.
The optical head described in the prior art is constructed using nonpolarizing optics, so that the polarization direction of the linearly polarized light emitted from the light source remains unchanged until it reaches the photodetector.
Accordingly, when the alignment direction of the liquid crystal in the liquid crystal panel is made to coincide with the polarization direction, if the optical recording medium is tilted from the normal in the radial direction relative to the optical axis, causing the phase distribution shown in FIG. 17 to occur at the best focal plane, the light spot focused on the recording surface of the optical recording medium and the light spot on the photodetector are substantially free from aberrations and stable servo signals and information can be obtained, since the light entering the liquid crystal panel is given the opposite phase distribution by external signals.
Here, for example, when the output of the light source is low or the reflectivity of the optical recording medium is low, or when the light transmitting efficiency of the optics is low, or when the light output on the optical recording medium has to be held low, such as when reading signals from an information erasable optical recording medium, the efficiency of light utilization decreases and the signal to noise ratio (S/N ratio) degrades. In view of this, recent optical heads employ polarizing optics to improve the efficiency of light utilization.
However, in a polarizing optical head using the liquid crystal panel having the above-described structure, since the liquid crystal panel is arranged so that the polarization direction of the linearly polarized light emitted from the light source coincides with the alignment direction of the liquid crystal, the aberration occurring in the forward path (the optical path from the light source to the optical recording medium) can be corrected, but the linearly polarized light in the return path (the optical path from the optical recording medium to the photodetector) is polarized at right angles to the light in the forward path.
Accordingly, if an external signal is applied to the liquid crystal panel, the refractive index does not change and the phase of the light remains unchanged. As a result, the aberration occurring in the return path cannot be corrected, and an offset occurs in the servo signal.
For the prior art optical head, no description is given of a servo signal detection method; here, assuming that the well known SSD method which detects spot size, specifically, the focus detection method using the anisotropic polarizing hologram described in Japanese Laid-open No. 8-77578, is employed as the detection method, the reason that an offset occurs in the servo signal will be described in detail below. FIG. 33(a) shows how the light spot is formed on the photodetector when the light spot on the recording surface of the optical recording medium is maintained in the correctly focused state. The output value (focus error signal FE) of the photodetector, when expressed using the symbols assigned to the plurality of split regions of the photodetector shown in FIG. 33(a), can be obtained by calculating the following equation (1).
FE=(S1axe2x88x92S1bxe2x88x92S1c)xe2x88x92(S1axe2x80x2xe2x88x92S1bxe2x80x2xe2x88x92S1cxe2x80x2)xe2x80x83xe2x80x83(Equation 1)
FIG. 33(a) shows the light spot on the photodetector under ideal conditions, but the light spot shifts in the radial direction by an amount m, as shown in FIG. 33(b), for example, on account of adjustment/assembly errors or variations in wavelength due to temperature changes. In this case, when the amounts of light falling on the respective regions of the photodetector are compared, S1a=S1axe2x80x2, S1b=S1bxe2x80x2, and S1c=S1cxe2x80x2, so that the focus error signal is zero (see equation 1). Further, when the optical recording medium contains a radial tilt angle, since the wavefront aberration is not corrected in the return path, as earlier described, the light spot on the photodetector will be as shown in FIG. 33(c).
Dotted lines show the sidelobes of the light spot due to coma. As for the sidelobes of the light spot due to the wavefront aberration, since a portion of the light spot is shifted outside the photodetector, when the amounts of light falling on the respective regions of the photodetector are compared
S1a less than S1axe2x80x2, S1b less than S1bxe2x80x2, S1c less than S1cxe2x80x2
Hence, an offset occurs in the focus error signal.
Firstly, the present invention has been devised in view of the above-described problems of the prior art relating to the radial and tangential tilt corrections, and it is an object of the invention to provide an optical element that can correct a plurality of aberrations simultaneously, and that achieves a thin and low cost construction by reducing the number of glass sheets by forming the pattern of the liquid crystal panel on the same side of the glass, and by also reducing the number of liquid crystal layers to one.
Secondly, in order to solve the prior art problem associated with the above-described polarizing optics, it is an object of the invention to provide an optical element, etc. capable of correcting aberrations in the return path.
Thirdly, it is a second object of the invention to achieve a compact, thin, and low cost construction for an optical head having an increased tilt margin by using the optical element of the invention.
To achieve the above objects, the optical element of the present invention comprises: a first aberration correcting electrode split into a plurality of regions; a second aberration correcting electrode split into a plurality of regions; an insulating film interposed between the first aberration correcting electrode and the second aberration correcting electrode; a counter electrode arranged substantially parallel to the aberration correcting electrodes; and a phase change layer interposed between the first aberration correcting electrode and the counter electrode. In this optical element, the phase of incident light can be changed using the phase change layer. With the provision of the plurality of aberration correcting electrodes, the wavefront aberration associated with radial tilt of an optical recording medium and the wavefront aberration associated with tangential tilt, for example, can be corrected simultaneously by using one phase change layer. Thus, the above optical element not only is thin in construction, but also is capable of correcting a plurality of aberrations simultaneously.
Preferably, in the above optical element, insulating film and aberration correcting electrodes, each split into a plurality of regions, are formed in multilayer structures on top of the second aberration correcting electrode. In that case, three or more aberrations can be corrected simultaneously.
Preferably, in the above optical element, the insulating film is formed from a solgel film. This facilitates the fabrication of the optical element.
Preferably, in the above optical element, the insulating film is formed from a material having substantially the same refractive index as the aberration correcting electrodes. This serves to reduce refractions at the interface between the insulating film and each of the aberration correcting electrodes.
Preferably, in the above optical element, the insulating layer has a thickness equal to an integral multiple of xcex/(2xc3x97N(insulating film) (where xcex is the wavelength of light incident on the optical element, and N(insulating film) is the refractive index of the insulating film). In that case, refractions at the interface between the insulating film and each of the aberration correcting electrodes can be reduced, whatever material is used for the formation of the insulating film.
One aspect invention of the present invention is an optical element comprising:
a phase change layer for changing the phase of light passing therethrough;
a plurality of aberration correcting electrodes, arranged on one side of said phase change layer, for correcting optical aberrations, each of said aberration correcting electrodes being split into a plurality of regions;
an insulating film interposed between said aberration correcting electrodes; and
a specific electrode arranged on the other side of said phase change layer, and wherein:
said aberration correcting electrodes, at least other than the outermost aberration correcting electrode as viewed from said phase change layer, have openings formed in prescribed positions, and
each of said aberration correcting electrodes, other than the innermost aberration correcting electrode, faces said phase change layer through said openings formed in other aberration correcting electrodes.
In the above optical element, the phase of incident light can be changed using the phase change layer. Since the phase change layer is provided between the aberration correcting electrodes, the wavefront aberration associated with radial tilt of an optical recording medium and the wavefront aberration associated with tangential tilt, for example, can be corrected simultaneously by using one phase change layer. Thus, the above optical element not only is thin in construction, but also is capable of correcting a plurality of aberrations simultaneously.
Preferably, in the above optical element, the plurality of aberration correcting electrodes and the counter electrode are formed using the same electrically conductive material. This facilitates the fabrication of the optical element. Further preferably, the electrically conductive material is an Indium Tin Oxide alloy (ITO, Indium Tin Oxide). This serves to reduce light losses at electrode portions.
Preferably, in the above optical element, the refractive index of the phase change layer changes according to an externally applied signal. With this provision, the phase distribution of the incident light can be changed by applying an external signal, to effect aberration correction. If liquid crystal is used for the phase change layer, the magnitude of the external signal can be reduced.
Preferably, in the above optical element, the thickness of the phase change layer changes according to an externally applied signal. With this provision, the phase distribution of the incident light can be changed by applying an external signal, to effect aberration correction.
If PLZT (transparent crystal having a perovskite structure of lead oxide, lanthanum, zirconium oxide, and titanium oxide) is used for the phase change layer, the thickness of the optical element itself decreases further since the phase change layer is a solid.
Preferably, the above optical element further includes an antireflective film for preventing reflections of incident light. This serves to prevent light losses due to reflections.
The optical head of the present invention utilizes the above-described optical element of the invention. Accordingly, when reproducing or recording information on an optical recording medium by means of light, optical aberrations can be corrected accurately.
The optical recording/reproduction apparatus of the present invention utilizes the above-described optical head of the invention, and controls the optical head based on signals output from the optical head and reproduces or records information on an optical recording medium. According, information can be reliably reproduced or recorded even when the optical recording medium is defective.
On the other hand, to achieve the above object concerning the polarizing optics, the optical element of the present invention comprises:
a first aberration correcting electrode split into a plurality of regions;
a second aberration correcting electrode split into a plurality of regions;
a first counter electrode arranged substantially parallel to the first aberration correcting electrode;
a second counter electrode arranged substantially parallel to the second aberration correcting electrode;
a first phase change layer disposed between the first aberration correcting electrode and the first counter electrode; and
a second phase change layer disposed between the second aberration correcting electrode and the second counter electrode, and wherein:
the first phase change layer changes the phase of linearly polarized light polarized in a prescribed direction, and the second phase change layer changes the phase of linearly polarized light polarized at right angles to the linearly polarized light polarized in the prescribed direction. Further, the phase change layer or layers are liquid crystal layers, and the prescribed direction corresponds to the alignment direction of an alignment film.
The above optical element is capable of changing the phase of incident light.
By forming the aberration correcting electrodes with the same split region pattern, wavefront aberrations associated with the tilt angle of the optical recording medium can be corrected using a single optical element even if the polarization direction of incident light is different.
Preferably, in the above optical element, the aberration correcting electrodes and counter electrodes are formed using the same electrically conductive material. This facilitates the fabrication of the optical element.
Further preferably, the electrically conductive material is an Indium Tin Oxide alloy (ITO, Indium Tin Oxide). This serves to reduce light losses at electrode portions.
Preferably, in the above optical element, the refractive index of the liquid crystal changes according to an externally applied control voltage signal. With this provision, the phase of the incident light can be changed by applying an external control voltage signal, to effect aberration correction. Further, since the liquid crystal is used, the magnitude of the external voltage signal can be reduced.
Preferably, the above optical element further includes an antireflective film for preventing reflections of incident light. This serves to prevent light losses due to reflections.
The optical head of the present invention utilizes the above-described optical element of the invention. According, if wavefront aberrations associated with the tilt angle of the optical recording medium are generated, optical aberrations can be corrected accurately when reproducing or recording information on the optical recording medium by means of light.
The above optical head comprises: converging optics for focusing light emitted from the light source onto the optical recording medium, and for converging light reflected from the optical recording medium; a K-quarter wave plate (where K is an odd number) for changing the polarization state of the light emitted from the light source; separating means for passing therethrough light polarized in a particular direction, and for separating light polarized in a direction perpendicular to the particular direction; and a photodetector for outputting a focus error signal and a tracking error signal for the optical recording medium, as well as an information signal, by receiving the light reflected from the optical recording medium and separated by the separating means.
By forming the aberration correcting electrodes with the same split region pattern, if wavefront aberrations associated with the tilt angle of the optical recording medium are generated when the polarization direction of incident light is different, optical aberrations can be corrected accurately when reproducing or recording information on the optical recording medium by means of light.
The optical recording/reproduction apparatus of the present invention uses the above-described optical head of the invention, and reproduces or records information on the optical recording medium. With this apparatus, information can be reliably reproduced or recorded even when the optical recording medium is defective.